The present invention relates to an image forming apparatus and method for forming an image on a predetermined printing medium and, more particularly, to an image forming apparatus and method suitable for printing a barcode on a printing medium.
In general, various printing apparatuses for printing on printing media (to be referred to as a printing sheet hereinafter) such as a paper sheet, cloth, plastic sheet, OHP sheet, and the like have been proposed, and they can include print heads based on various printing schemes, e.g., a wire-dot scheme, thermal scheme, thermal transfer scheme, and ink-jet scheme.
Of these schemes, an ink-jet scheme which is one of a class of low-noise, non-impact schemes that eject ink continous schemes (including a charge particle control scheme and spray scheme) and on-demand schemes (including a piezoelectric scheme, spark scheme, bubble-jet scheme) depending on their ink droplet forming methods and generation methods of ejection energy.
In the continous scheme, ink is continously ejected, and charges are given to only a required number of ink droplets. The charged ink droplets become attached to a printing sheet, but other ink droplets are wasted. In contrast to this, in the on-demand scheme, since ink is ejected only as required for printing, ink is not wasted, and the interior of the apparatus is not contaminated with ink.
In the on-demand shcme, since ink ejection is repetitively started and stopped, the response frequency is lower than that in the continous scheme. For this reason, the on-demand scheme realizes high-speed printing by increasing the number of nozzles that ejext ink droplets. Hence, most of currently commercially available printing apparatuses adopt the on-demand scheme, and printing apparatus with such ink-jet print head is commercially available in the form of output means of an information processing system, e.g., a copying machine, facsimile apparatus, wordprocessor, a printer as an output terminal of a personal computer or the like, and the like, since it can attain high-density, high-speed printing.
An ink-jet printing apparatus commonly comprises a print head, an ink tank that supplies ink to the print head, a conveyance means for conveying a printing sheet, and a control means for controlling these components. A carriage that mounts the print head for ejecting ink droplets from a plurality of orifices is serially scanned in a direction perpendicular to the conveyance direction of a printing sheet, and the printing sheet is intermittently conveyed by an amount equal to a printing width in a non-printing state. By repeating such operations, a significant, two-dimensional image is printed. This printing method prints by ejecting ink onto a printing sheet in correspondence with a print signal, and is widely used as a silent printing method with low running cost.
When a full-line print head on which nozzles for ejecting ink are arranged on a line in correspondence with the paper width of a printing sheet is used, printing for the paper width is attained by continuously conveying the printing sheet in a direction perpendicular to the nozzle array of the print head. With this print head, higher-speed printing can be accomplished.
Furthermore, a color ink-jet printing apparatus forms a color image by overstriking ink droplets ejected by a plurality of color print heads. In general, in order to perform color printing, three print heads corresponding to three primary colors, i.e., yellow (Y), magenta (M), and cyan (C), or four print heads corresponding to black (Bk) in addition to the above three primary colors, and ink tanks corresponding to the individual heads are required. Recently, an apparatus which mounts such three or four color print heads and can form a full-color image has been put into practical use.
Moreover, in the FA (factory automation) and SA (store automation) fields as well, demand has arisen for dedicated printing apparatuses since they can timely output commodity management labels/tags printed with color commodity pictures, distribution labels that classify destinations by colors, and POP and shelf display labels/tags in retail stores, convenience stores, and the like. Such printing apparatus comprises a barcode generator since it must print barcodes for management in the individual fields.
The demand for the ink-jet printing apparatuses as excellent printing means is increasing in various industrial fields (e.g., apparel industry), and also, further improvement of image quality is being sought.
As energy generation means for generating energy for ejecting ink in an ink-jet print head, an electromechanical energy conversion element such as a piezoelectric element or the like is used, as described above, or an electro-thermal conversion element having a heating resistor is used to heat ink.
Of such means, a print head (so-called bubble-jet head) that ejects ink using heat energy (using a film boiling phenomenon) can attain high-resolution printing since the ink orifices can be arranged at high density.
In order to achieve stably readable barcode printing by the ink-jet print head arranged in the ink-jet printing apparatus with the above arrangement, stable ejection and a constant line thickness ratio of barcodes must be maintained to print barcodes that comply with barcode standards.
Such requirements can be met by, e.g., temperature control of the heads to stabilize the ejection amount.
However, in the distribution and FA fields, and the like, a large quantity of printing is often done per job, and it is hard to stabilize the ejection amount by the conventional temperature control alone. As the number of prints per job becomes larger, the ejection amount increases, and the black bars of barcodes become thicker. As a result, stable reading precision cannot be assured.
In general, in a dedicated barcode printing apparatus that exclusively prints barcodes, a single print head is arranged, and prints barcodes by a monochrome thermal transfer or ink-jet method. Even when a versatile printing apparatus is used, barcodes are normally printed using a single color. Hence, in a color printing apparatus having a plurality of heads as well, barcodes are normally printed using a single color (in general, black).
In barcode printing by a print head for printing a barcode, troubles inherent to the printing schemes (e.g., disconnection of a head, skewing of a ribbon, or the like in the thermal transfer scheme; ejection errors of a head, mislanding of ink, or the like in the ink-jet scheme) are fatal. That is, when a barcode printed using the printing apparatus that has suffered such troubles is read using a barcode reader, reading errors occur or such barcode cannot be read. For this reason, in the above-mentioned conventional barcode printing method, barcode printing is disabled when such troubles have occurred.